An epiphenomenon is a secondary phenomenon that occurs alongside a primary phenomenon.

Often, a causal relationship between the phenomena is implied: the epiphenomenon is a consequence of the primary phenomenon. In medicine, this relationship is typically not implied: an epiphenomenon may occur independently, and is merely called an epiphenomenon because it is not the primary phenomenon under study. (A side-effect is a specific kind of epiphenomenon that does occur as a consequence.)